MC33368
http://onsemi.com
8
Multiplier
A single quadrant, two input multiplier is the critical
element that enables this device to control power factor. The
ac haversines are monitored at Pin 5 with respect to ground
while the Error Amplifier output at Pin 4 is monitored with
respect to the Voltage Feedback Input threshold. A graph of
the Multiplier transfer curve is shown in Figure 2. Note that
both inputs are extremely linear over a wide dynamic range,
0 to 3.2 V for Pin 5 and 2.5 to 4.0 V for Pin 4. The Multiplier
output controls the Current Sense Comparator threshold as
the ac voltage traverses sinusoidally from zero to peak line.
This has the effect of forcing the MOSFET on−time to track
the input line voltage, thus making the preconverter load
appear to be resistive.
Pin 6 Threshold [ 0.55
ǒ
V
Pin 4
–V
Pin 3
Ǔ
V
Pin 5
Zero Current Detector
The MC33368 operates as a critical conduction current
mode controller, whereby output switch conduction is
initiated by the Zero Current Detector and terminated when
the peak inductor current reaches the threshold level
established by the Multiplier output. The Zero Current
Detector initiates the next on−time by setting the R
S
Latch
at the instant the inductor current reaches zero. This critical
conduction mode of operation has two significant benefits.
First, since the MOSFET cannot turn−on until the inductor
current reaches zero, the output rectifier’s reverse recovery
time becomes less critical allowing the use of an inexpensive
rectifier. Second, since there are no deadtime gaps between
cycles, the ac line current is continuous thus limiting the
peak switch to twice the average input current
The Zero Current Detector indirectly senses the inductor
current by monitoring when the auxiliary winding voltage
falls below 1.2 V. To prevent false tripping, 200 mV of
hysteresis is provided. The Zero Current Detector input is
internally protected by two clamps. The upper 10 V clamp
prevents input overvoltage breakdown while the lower
−0.7 V clamp prevents substrate injection. An external
resistor must be used in series with the auxiliary winding to
limit the current through the clamps to 5.0 mA or less.
Current Sense Comparator and RS Latch
The Current Sense Comparator R
S
Latch configuration
used ensures that only a single pulse appears at the Drive
Output during a given cycle. The inductor current is
converted to a voltage by inserting a ground−referenced
sense resistor R7 in series with the source of output switch.
This voltage is monitored by the Current Sense Input and
compared to a level derived from the Multiplier output. The
peak inductor current under normal operating conditions is
controlled by the threshold voltage of Pin 6 where:
I
pk
+
Pin 6 Threshold
R7
Abnormal operating conditions occur when the
preconverter is running at extremely low line or if output
voltage sensing is lost. Under these conditions, the Current
Sense Comparator threshold will be internally clamped to
1.5 V. Therefore, the maximum peak switch current is:
I
pk(max)
+
1.5 V
R7
With the component values shown in Figure 16, the
Current Sense Comparator threshold, at the peak of the
haversine, varies from 110 mV at 90 Vac to 100 mV at
268 Vac. The Current Sense Input to Drive Output
propagation delay is typically 200 ns.
Timer
A watchdog timer function was added to the IC to
eliminate the need for an external oscillator when used in
stand alone applications. The Timer provides a means to
automatically start or restart the preconverter if the Drive
Output has been off for more than 385 ms after the inductor
current reaches zero.
Undervoltage Lockout and Quickstart
The MC33368 has a 5.0 V internal reference brought out
to Pin 1 and capable of sourcing 10 mA typically. It also
contains an Undervoltage Lockout (UVLO) circuit which
suppresses the Gate output at Pin 11 if the V
CC
supply
voltage drops below 8.5 V typical.
A Quickstart circuit has been incorporated to optimize
converter startup. During initial startup, compensation
capacitor C1 will be discharged, holding the Error Amplifier
output below the Multiplier’s threshold. This will prevent
Drive Output switching and delay bootstraping of capacitor
C4 by diode D6. If Pin 4 does not reach the multiplier
threshold before C4 discharges below the lower SMPS
UVLO threshold, the converter will hiccup and experience
a significant startup delay. The Quickstart circuit is designed
to precharge C1 to 1.7 V. This level is slightly below the
Pin 4 Multiplier threshold, allowing immediate Drive
Output switching.
Restart Delay
A restart delay pin is provided to allow hiccup mode fault
protection in case of a short circuit condition and to prevent
the SMPS from repeatedly trying to restart after the input
line voltage has been removed. When power is first applied,
there is no startup delay, but subsequent cycling of the V
CC
voltage will result in delay times that are programmed by an
external resistor and capacitor. The Restart Delay, Pin 2, is
a high impedance, so that an external capacitor can provide
delay times as long as several seconds.
If the SMPS output is short circuited, the transformer
winding, which provides the V
CC
voltage to the control IC
and the MC33368, will be unable to sustain V
CC
to the
control circuits. The restart delay capacitor at Pin 2 of the
MC33368 prevents the high voltage startup transistor within
the IC from maintaining the voltage on C4. After V
CC
drops
below the UVLO threshold in the SMPS, the SMPS
switching transistors are held off for the time programmed
by the values of the restart capacitor (C9) and resistor (R8).
In this manner, the SMPS switching transistors are operated
